linux-kernel - Re: [PATCH] bcache: add cond_resched() in __bch_cache

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Date:   Fri, 8 Mar 2019 08:35:17 +0800
From:   Shile Zhang <shile.zhang@...ux.alibaba.com>
To:     Vojtech Pavlik <vojtech@...e.com>, Coly Li <colyli@...e.de>
Cc:     Kent Overstreet <kent.overstreet@...il.com>,
        linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bcache: add cond_resched() in __bch_cache_cmp()


On 2019/3/7 23:44, Vojtech Pavlik wrote:
> On Thu, Mar 07, 2019 at 11:36:18PM +0800, Coly Li wrote:
>> On 2019/3/7 11:06 下午, Shile Zhang wrote:
>>> On 2019/3/7 18:34, Coly Li wrote:
>>>> On 2019/3/7 1:15 下午, shile.zhang@...ux.alibaba.com wrote:
>>>>> From: Shile Zhang <shile.zhang@...ux.alibaba.com>
>>>>>
>>>>> Read /sys/fs/bcache/<uuid>/cacheN/priority_stats can take very long
>>>>> time with huge cache after long run.
>>>>>
>>>>> Signed-off-by: Shile Zhang <shile.zhang@...ux.alibaba.com>
>>>> Hi Shile,
>>>>
>>>> Do you test your change ? It will be helpful with more performance data
>>>> (what problem that you improved).
>>> In case of 960GB SSD cache device, once read of the 'priority_stats'
>>> costs about 600ms in our test environment.
>>>
>> After the fix, how much time it takes ?
>>
>>
>>> The perf tool shown that near 50% CPU time consumed by 'sort()', this
>>> means once sort will hold the CPU near 300ms.
>>>
>>> In our case, the statistics collector reads the 'priority_stats'
>>> periodically, it will trigger the schedule latency jitters of the
>>>
>>> task which shared same CPU core.
>>>
>> Hmm, it seems you just make the sort slower, and nothing more changes.
>> Am I right ?
> Well, it has to make the sort slower, but it'll also avoid hogging the
> CPU (on a non-preemptible kernel), avoiding a potential soft lockup
> warning and allowing other tasks to run.
>
Yes, there is a risk that other tasks have no chance to run due to sort 
hogging the CPU, it is harmful to some schedule-latency sensitive tasks.
This change just try to reduce the impact of sort, but not a performance 
improvement of it. I'm not sure if a better way can handle it more 
efficiency.

Thanks,

Shile